14 people who turned their hobbies into million- and billion-dollar careers

As the saying goes, "Do what you love, and you'll never work another day in your life."

These entrepreneurs did just that, and they've made millions!

From Jennifer Hyman and Jennifer Fleiss -- fashion-savvy business school classmates who launched the highly successful apparel rental service Rent the Runway -- to Mark Zuckerberg, who started the platform that became Facebook as a side project, here are 14 people who turned their hobbies into million- and billion-dollar careers.